Colin (00:45): However after the second world war the decision was taken to up sticks and leave Greenwich due to the increasing levels of light pollution from nearby London. Despite the departure of the Astronomers Royal, the observatory continues to make history. Let's head up onto the roof of the south building behind us and find out how.

Emily (01:02): This is what the view looked like from the roof of the south building in the mid 1890s. The buildings in the foreground are the old magnetic observatory and the magnetic offices. I'm sure you'll agree that the view today is very different. It is now dominated in this area by this huge, 45 ton bronze cone. What's more interesting about that is that beneath it lays London's only planetarium - the Peter Harrison Planetarium.

Colin (01:26): Construction began on this ambitious project in the autumn of 2005. It was part of a £17.7 million redevelopment of the south end of this UNESCO World Heritage site. Here's an insight into how this mega-structure was built.

Emily (02:04): Since its official opening in May 2007, everyday a stream of visitors walks past the cone and wonders at its peculiar shape. But its shape is no mere coincidence. In fact, each side of it has some kind of astronomical significance. To find out more I'm now going to join Dr Marek Kukula who is the public astronomer here at the observatory.

Emily (02:28): So Marek, what is the unusual shape all about?

Marek (02:30): Well you're right, it's very striking. We're standing infront of the south face now which is angled at 51.5° to the ground which means that it's parallel to the rotation axis of the Earth. So that groove you can see on the south side actually points straight up at the north celestial pole and the pole star Polaris. 51.5° is also the latitude here at Greenwich so that's the significance of that. The building is oriented in the north-south direction which means it is aligned with the Prime Meridian which passes just a few metres over there to the west.

Emily (03:04): So shall we go around the other side because there are some more things around there?

Emily (03:12): So we're now on the other side of the cone. What is the shape all about?

Marek (03:16): Yes that's right. We're now on the north side of the cone. You can tell that because it's in shadow because it's about the middle of the day so the shadow is pointing north. The north face is actually vertical so it points directly up towards the local zenith point here at Greenwich. You can see that the top of the cone has been sliced off at a particular angle. The significance of that is that that circular plane at the top is parallel to the celestial equator. It's got a reflective coating so reflects the northern hemisphere of the sky and, as visitors come into the site from over here, the first things they see is the sky - the clouds during the day or the stars at night - reflected in the top of the cone. Hopefully that will encourage them to go down into the planetarium underneath.

Emily (03:59): So this thing covers the planetarium.

Marek (04:00): That's right. The planetarium dome is inside the cone there and that's what we project all the star shows onto. The seating area for visitors is down underground so that they can look up into the inside of the dome.

Emily (04:12): Well thanks for talking to us Marek. Let's now head down into the planetarium.

Emily (04:17): Now I bet you've been wondering why we've had such exclusive access to the planetarium. Well it turns out that one of the Jodcast team actually works here. Let's go find out who that is.

Emily (04:33): So how long have you been working here?

Colin (04:34): Since April really. I volunteered here last summer and then bugged them until they gave me a job. I've been here on a freelance basis since April.

Emily (04:43): And it's been open since May 2007?

Colin (04:45): Yes. Just over a year. So originally, as you saw upstairs, it was a building site so in here there was a big hole in the ground. Over time it got built up further so now we have a 118 seat auditorium. We get visitors to come in and they get to sit back in these comfy chairs and look up and see all the things that the night sky has to offer them.

Emily (05:04): Is it just Londoners that come here?

Colin (05:06): Not at all. We get people from all over the world; most continents.

Emily (05:12): So what sort of shows is it that you have then?

Colin (05:14): There's a good mix of pre-recorded shows and live shows. So we've got things like black holes also the life cycle of stars. Also live shows where astronomers like myself put up, onto the planetarium dome, the night sky as it would appear that evening. A bit of a guided tour really.

Emily (05:32): Shall we go take a look at how it all works then?

Colin (05:33): Sure. I'll show you the back.

Emily (05:37): So this is where all the magic happens then?

Colin (05:39): Yes. This is the back where we control the planetarium. So for the live shows I've got a set of buttons here which have all the main constellations from the northern hemisphere. Both the pictures - the artwork - and the lines as well so I can choose what I put up on the dome. Here are all the light controls so I can slide the lights up and down and change the colours of the dome. Basically, for the live shows, the control over what happens in them is mostly mine. You set up a main frame for it and then you have 24 and a half minutes to talk about what's in the sky that night. After 24 and a half minutes the computer cuts in, takes control away from you and does its pre-recorded end sequence which I still commentate over the top of.

Emily (06:24): You just get to make up these live shows then?

Colin (06:27): There's no script as such but obviously we want to make sure that people that come in and see the shows can go back out again to the night sky and replicate it for themselves. So we make sure that we show them things like The Plough, Cassiopeia; big constellations and obvious ones, so they can use them as landmarks as a way across the sky. We also like to use stories as well to try to get the kids interested; the mythology behind some of the constellations. Do you want to have a look out the back and see some of the hardware that runs all this equipment.

Colin (07:01): So this is the meat of the hardware that runs the planetarium. Behind me with the green lights is the audio racks. That's what runs the sound in the planetarium. There is also some soundproofing in the walls so it's a bit like a music studio so you get the full audio experience. Also here we've got 16 different computers which run each one sixteenth of the planetarium dome and then they get blended seemlessly to make one whole dome image. You might be able to hear and feel the air conditioning in here. It's pretty cold. It is because there is so much computer equipment in here and it gets so hot that we need the air conditioning so that it doesn't burn out.

Emily (07:39): Really impressive.

Emily (07:41): We've had a really interesting time at the Royal Observatory, Greenwich over the past few days and we've learnt a lot. We've looked at why the site was founded and how John Harrison solved the longitude problem using his clocks. We've also had a good look at the planetarium and how all that works.

Colin (07:55): It's been really good to see how the Royal Observatory has gone from Britain's first purpose built scientific research facility to now being at the cutting edge of astronomy education. Thanks for watching.